High and low temperature behavior of Ohmic contacts to AlGaN/GaN heterostructures with a thin GaN cap

V/Al/V/Ag contacts that provide low specific contact resistances as well as smooth surfaces and contact edges on AlGaN/GaN heterostructures were recently introduced. In the present study, the contacts are examined in greater detail, including an evaluation of their high temperature stability and their specific contact resistance as a function of measurement temperature. The contacts provide a low specific contact resistance of 5.2× 10-7 cm2 and contact resistance of 0.15 mm after aging for 10 h at 500 °C in flowing N2. Furthermore, cross-sectional transmission electron microscopy (TEM) of the aged V/Al/V/Ag contacts shows little change in phase formation compared to the annealed contacts that were not aged. Beneath the annealed V/Al/V/Ag metallization, the AlGaN layer is left intact. On the other hand, cross-sectional TEM images of previously reported V/Al/V/Au and Ti/Al/Ti/Au contacts reveal that both of the Au-bearing metallizations react through a portion of the AlGaN layer and directly contact the underlying GaN. By comparing these TEM images with the observed temperature dependence of the specific contact resistance of all of the contacts measured from 77 to 573 K, conclusions regarding current transport mechanisms were drawn.